The overall goal of this project is to explore the mechanisms that are involved in the development of neutralizing antibodies and protective immunity during vaccination and during the immediate post-challenge time frame. This knowledge will be used to design improved vaccine strategies to offer protection from infection, not only protection from disease. These experiments will be performed in the SHIV(SF162P) model of moderate pathogenesis in the pigtailed macaque (M. nemestrina). Virus neutralizing antibodies can be elicited by some limited number of vaccination strategies, but these have been typically low in quality and quantity. In the course of this project, we plan to develop immunogens that can elicit broad cellular immunity and broad, cross-clade protective humoral immunity. Naturally occurring HIV Envelope variants (quasispecies Envelope variants) will be used in combination with the other structural and regulatory genes encoded by SHIV in vaccine challenge experiments in macaques. Vaccines will be delivered by recombinant vaccinia virus, DNA, and recombinant oligomeric gp140 proteins. There are four aims: AIM 1. Develop quasispecies vaccines based on the viral variants arising in the course of SHIV(SF162P) infection of M. mulatta and M. nemestrina and HIV subtype A infection of human patients from Kenya (Project 1). Choose subjects that developed broadly reactive neutralizing antibodies. Select variants that represent Envelopes present during the development of the quasispecies and that are neutralized by sera from SHIV-infected macaques, human HIV + sera, and human mAbs that neutralize primary HIV-1 isolates. AIM 2. Examine the rote of quasispecies Envelopes derived from SHIV(SF162P) in eliciting broadly neutralizing antibodies. By presenting combined and individual quasispecies variant Envelopes to the immune system during vaccination, determine which pattern of exposure to the variants elicits qualitatively different responses compared with the exposure to a single Envelope protein. Choose the optimal vaccination strategy that offers the best protection from infection or disease following SHIV(SF162P) challenge. AIM 3. Compare subtype A Envelope quasispecies vaccines and subtype B subtype quasispecies vaccines for the ability to provide same-clade and cross-clade protection from a SHIV-A challenge. Measure correlates of protection from infection and disease.